The present invention relates generally to methods and apparatus for treating fluids, and specifically relates to methods and apparatus for treating fluids through contact with a contact media. The method and apparatus of the present invention are particularly suitable for use in ion exchange operations where a fluid is contacted with a fluidizable ion exchange media such as a resin, or with filtration or absorption operations where the fluids contacted with a media such as activated carbon.
Ion exchange processes for treating fluids are well known. Such ion exchange operation may include, for example; "water softening", deionization, de-alkylizing, desilicizing, and organic scavenging. With respect to the present invention, these ion exchange processes will for convenience be discussed in terms of water treatment. It should be clearly understood, however, that the methods and apparatus of the present invention may be utilized in the treatments of many other fluids.
Typically, an ion exchange process is affected by flowing the water through a vertical column of an ion exchange contact media, typically a resin. As the water contacts the resin, ions in the water will be attracted to the resin from the water. One type of resin may be utilized to remove cations from the water and a second type of resin may be utilized to remove anions from the water. Preferably, the separate resins will be contained in separate beds. However, conventional techniques of water deionization include the use of two resins mixed in a single bed.
The softening of water by ion exchange is accomplished by replacing the calcium and magnesium ions in the water by an equivalent number of sodium ions from the resin. Resin in the bed will contain only a finite number of exchangeable sodium ions. This number defines the "capacity of the resin". When the capacity of the resin has been exhausted, i.e., when all of the exchangeable sodium ions on the resin have been replaced by calcium and magnesium ions from the water, the resin must be regenerated back to the sodium form. This regeneration is typically accomplished by passing a sodium chloride solution (a brine) through the resin. Additionally, the resin will be rinsed to remove excess brine, and will be backwashed to remove particulate matter which may have accumulated in the resin during the ion exchange step ("the service cycle").
As water flows through a bed of resin, the majority of the ion exchange will take place in the portion of the resin which is first contacted by the fluid. In an ion exchange system where the fluid flows downwardly through the bed, this exchange creates an "exhausted band" of exhausted resin which expands downwardly through the bed as the operation continues. When the band approaches the bottom of the resin bed, the bed must be regenerated as discussed above.
Additionally, a vertical column of resin operating in a service cycle has an exchange zone, or active band, starting at the top and moving down through the bed of resin. The width of the active band varies with certain operating parameters of the system. For example, as the service flow is increased the active band will spread out. The resin bed must be removed from service and regenerated before the active band reaches the bottom of the column to prevent leakage of the ions being removed from the fluid. This prevents full utilization of the resin since there is resin not fully exhausted in and below the active band. When a column of resin in normal operation is regenerated, sufficient regenerant (salt in the case of water softening) is used to regenerate the entire volume regardless of what percent of the bed was actually exhasted.
Many conventional vertical ion exchange columns are designed to function both as a column for the service cycle, i.e., for the initial ion exchange process, and for the regeneration cycle, and therefore also include provisions for backwashing, regeneration, and rinsing of the resin. This structure requires the fluid influent to be shut off from the column while the regeneration operation takes place, thereby interrupting the supply to service of treated fluid. When an uninterrupted supply of treated fluid is required, a second vertical column is typically provided. This second column will be regenerating during the service cycle of the first column and vice versa. Conventional columns typically include a shell to contain the resin, a support for the resin, and means for distributing flow both upwardly and downwardly through the resin, (for both the service cycle and the regeneration and backwashing cycles). The shell must have sufficient space above the resin to allow the resin to expand during the backwashing operation. Valves and controls are typically necessary to bypass "raw" (untreated) fluid around the column during the regeneration cycle, to inject the regeneration fluid into the column, and to reverse the direction of fluid flow for backwashing.
Conventional vertical columns may include several disadvantages. Where a single large resin bed is utilized, fluid has a tendency to channel through the bed during periods of low flow rate, thereby reducing the effective contact of the fluid with the resin. Additionally, the requirement of additional space above the resin bed to facilitate the backwashing operation adds cost to the vessel. Where uninterrupted service is required and a second unit is provided, the additional unit adds significant cost and size to the unit. A significant factor in this cost is that a control valve must be provided to switch fluid flow from one vessel to the other. This control valve must be large enough to provide a significant flow of the influent into the column without placing an excessive pressure drop in the system. Large control valves of this type typically contribute a significant portion of the cost of conventional ion exchange units. These valves still often place an undesireable pressure drop in the system.
If only one column is provided, in typical conventional systems, not only must the flow of treated water be interrupted, but untreated water must be used with the unit itself for the backwashing, regeneration, and rinsing operations. This use of untreated water will, in itself, decrease the operating efficiency of the contact media regeneration, and will therefore similarly decrease the efficiency of the ion exchange process.
Because of the deficiencies discussed above, several attempts have been made to devise methods and apparatus for an uninterrupted or, continuous ion exchange process in a single column. Typically, these processes involve the movement of the contact media downwardly through the column or ion exchange vessel while the fluid flows upwardly through the column. In some cases, the resins are actually fluidized, or suspended, in the fluid flow. This upward flow, and especially fluidization, typically provide less than optimal ion exchange. A major factor in the efficiency of ion exchange process is the physical contact of the water molecules with the resin. With a downward fluid flow, both gravity and the influent flow serve to compact the resin into a tightly formed bed. This compacting of the bed forces the fluid to flow closer to the resin beads, causing surface effects on the water and forcing fluid to flow into the pores of the beads. This compacting of the bed, therefore, increases both contact efficiency and the bed capacity. In contrast, an upward flow, as found in the prior art, causes the resin to expand, as noted above, sometimes to the point of fluidization. This unpacked state of the resin causes a reduction in contact with the water. Additionally, even when a system is designed to operate with the resin not in a state of fluidization, flow rate surges must be prevented to avoid the fluidization.
As indicated above, conventionally proposed continuous ion exchange methods and apparatus typically move the resins downwardly through the exchange vessel. When each portion of the resin reaches a predetermined location in the vessel, the resin is removed and regenerated in a separate vessel. Therefore, for efficient use of the system and the contact media, the rate of travel of the media must be regulated in response to the rate of flow of the influent.
Conventionally proposed methods and apparatus for continuous ion exchange typically provide for contacting the fluid with a single resin in a vessel. However, for operations such as water deionization or demineralization, the water is typically contacted with two ion exchange resins in two stages. In the first stage, the water contacts a first resin which will attract the cations from the water and replace them with hydrogen ions. This first resin is typically regenerated with an acid. In the second stage, the fluid is contacted with a second resin which attracts the anions from the water and replaces them with hydroxide ions. This second resin is typically regenerated with a base, such as sodium hydroxide. The hydrogen ions from the first stage and the hydroxide ions from the second stage combine to produce water. Similarly, fluid may be contacted with selected resins for other treatment operations, such as dealkylization, etc. Each of these resins will require a selected regenerization fluid. The fluid must, therefore, be treated with the different resins in different columns or with a mixture of the two resins. If a mixture of the resins is utilized, then the resins must be separated prior to regeneration.
Additionally, activated carbon is often utilized in fluid treatment operation. The activated carbon may be used to remove gases and other organic impurities giving taste and odor to drinking water. Activated carbon may be used as a pretreatment for water supplied to water softening systems to prevent organic fouling of the resin beds, and may also be utilized in waste water treatment. Although activated carbon is not susceptible to being "regenerated" per se, the carbon periodically requires backwashing to remove suspended matter and to re-grade the bed. With conventionally proposed continuous water treatment operations, an activated carbon section must be contained within a separate vessel.
Accordingly, the present invention provides a new method and apparatus for the continuous treatment of fluids in a single column wherein the contact media need not be treated until it is completely exhausted. Additionally, multiple contacting media may be utilized for different types of ionic exchange, absorption or filtration in the column, and each may be regenerated without interrupting the continuous service flow. Also, this continuous fluid treatment can be performed with an optimal downward fluid flow. The invention, thus, significantly overcomes the deficiencies presented by the prior art.